Compositions for coloring a thermoplastic resin include a powdery dry color in which a pigment and a dispersant therefor are blended, a liquid color or paste color in which a pigment is dispersed in a dispersant which is liquid at ambient temperature, and a master batch in the form of pellets, flakes, or beads, in which a pigment is dispersed in a resin which is solid at ambient temperature. These coloring compositions are properly used in accordance with their use to make the most of their characteristics. Among them, master batches are used conventionally from the viewpoints of easiness in handling and preservation of the working environment during use. Therefore, master batches have nowadays come to require even higher pigment dispersibility and distribution properties than before as thermoplastic resins are molded more precisely and quicker, as well as have properties of high pigment concentrations and reduced influence upon various characteristics such as heat resistance and strength of the thermoplastic resins to be colored.
When a high level of pigment dispersion is required e.g., in cases where thermoplastic resins are spun at a high speed to a diameter of 10 and several microns or processed into films, it may happen that conventional pigment dispersibility of master batches is insufficient. That is, problems due to poor pigment dispersion occur, such as breakage of threads during spinning, plugging of filters of melt spinning machines, and shaping deficiency during filming. In order to solve these problems and improve the dispersibility of master batches, it is a conventional approach to use, as dispersants for providing dispersibility, one or more of stearic acid, zinc stearate, magnesium stearate, aluminum stearate, calcium stearate, lithium stearate, ethylene bisamide, polyethylene wax, polypropylene wax, or waxes composed of derivatives thereof, such as acid-denatured products. In addition, attempts have been made for improving methods of processing a master batch or for enhancing pigment dispersibility by use of a powerful kneader. However, sufficient pigment dispersibility to solve the above-mentioned problems was not obtained.
In addition, in the field of large-size injection molding in which colored pellets have conventionally been used, color unevenness and flow marks have become the problem in coloring molded products, as master batches are more frequently used for coloring. In blow molding and film forming in which master batches are conventionally used for coloring, plasticizing, blending, and kneading are all carried out in an extruder of the molding machine. On the other hand, in injection molding machine, plasticizing, blending, and kneading steps are carried out in a cylinder in which a screw may move back. However, kneading power in the cylinder is not sufficient as compared with extruders. Moreover, kneading power decreases, as molding cycle time is shortened and molding resins have lower viscosities. As a result, color unevenness tends to occur in the surface of resulting molded products.
In addition, a master batch comes to be added to a resin to be colored in reduced amounts after development of high concentration master batches in which the pigment content in master batches is increased so as to reduce costs needed for coloring. As a result, a chance that color unevenness and flow marks occur has increased. Although this problem is common among a variety of thermoplastic resins, it is especially outstanding in polypropylene resins which are frequently used in recent years in the fields of household appliances and automobile parts. Therefore, a solution of the problem is needed without delay. A conventional approach for solving this problem is one of increasing the amount of a dispersant among the three primary components of a master batch (a pigment, a dispersant, and a base resin), or of reducing the melt viscosity of a master batch by, for example, using a base resin which has a viscosity smaller than that of a resin to be colored.
However, polypropylene resins filled with inorganic fillers or having a melt flow rate (hereinafter referred to as MFR) in excess of 25 for facilitating a thin wall molding tend to generate color unevenness or flow marks. Therefore, this disadvantage must be overcome.
Conventionally, master batches are prepared by premixing a composition of a dry powdery pigment, a dispersant, and a base resin in a Henschel mixer, or the like, introducing the composition into a batch-type kneader such as a three roll mill or a kneader, melt-kneading it for a long time, pulverizing a resulting pigment dispersion using a crusher to a size that enables supply to a uniaxial extruder, and then by pelletizing them with the uniaxial extruder. However, since powdery pigment once dried contains numerous agglomerated particles including coarse secondary, tertiary, and higher grade cohesion particles, it is very difficult to re-disperse these coarse agglomerated particles at a size of not more than several micrometers. Moreover, the process is a batch-type and employs many steps, automation and reduction of labor are desired. In addition, improvement in a working environment in which dry pigments are scattered by the use of dry powdery pigments is also desired.
An object of the present invention is to overcome the above-mentioned various drawbacks and to provide an efficient process for preparing a resin composition (a master batch) for coloring a thermoplastic resin, which does not cause 5% or more decrease in mechanical strength of the thermoplastic resin, for example, tensile strength, bending strength, and impact strength thereof, which has excellent pigment dispersion ability, which is capable of providing a uniform coloring without causing color unevenness, the process providing a good working environment.